Apparatus for manufacturing asphalt paving are well known. Asphalt pavement, more commonly known as blacktop, is produced by mixing asphalt with sand and gravel and heating the mixture to a temperature around 300.degree. F. Asphalt is the black material which binds the aggregate together. Asphalt is derived from crude oil along with the more familiar hydrocarbons of gasoline, kerosene and heating oil. At the paving temperature of 250.degree. to 350.degree., asphalt is a viscous liquid which coats the aggregate and sand and, upon cooling, binds the aggregate and sand together to form a tough wear-resistant pavement known as blacktop.
Asphaltic paving material may also be manufactured from recycled material in whole or in part. Recycled blacktop, also known as Recycled Asphalt Pavement or "RAP", is broken up into small chunks and heated. Where aging of the recycled pavement has removed important components called volatiles from the asphalt, these may be replaced with light asphalts or rejuvenating fluid containing a mixture of lighter hydrocarbons to obtain the design percentage of asphalt in the mix.
Heating recycled asphalt presents a problem when designing an asphaltic pavement plant. The reused asphalt paving material must be heated to soften and melt the blacktop so it may be made workable to enable it to be modified to obtain a desired standard. However, if the asphalt is subjected to high temperatures in the heating process such as by exposure to direct flame, the asphalt will char. Charring of the asphalt is undesirable because it produces smoke and polluting fumes and reduces the ability of the asphalt to cement the aggregate together.
One method of producing asphaltic aggregates employing recycled blacktop is to heat virgin aggregate, which is not subject to charring, to 600.degree. or 700.degree. F., then mixing it with the RAP. The mixture of virgin aggregate and RAP produces a mixture with a desirable paving temperature of 250.degree. to 350.degree. F. which is mixed with sufficient asphalt to form the finished paving material. This approach has two drawbacks: 1) even 600.degree. or 700.degree. F. aggregate results in some charring and smoking of the RAP. Secondly, this limits the percentage of reused blacktop to 50% or 60% of the total mixture. However, it is desirable for reasons of cost and oil conservation to process mixtures containing up to 100% RAP.
Another desirable attribute of an asphalt paving plant is a high production rate to meet requirements for modern paving projects which require at least 100 tons per hour and preferrably 400 to 600 tons per hour. Conventional paving plants achieve high production rates by employing rotating drum heaters which employ a large centrally located burner disposed in one end of the drum which heats the virgin aggregate or aggregate and RAP as they move down the axis of the rotating drum. This type of asphalt plant can have the undesirable effect of overheating the asphalt in the recycled material.
Known methods of dealing with the overheating of the recycled material and any asphalt which is added to the aggregate is to add these materials to the drum away from the heated end. This limits the amount of recycled material which can be added and still reach a final temperature of around 300.degree. F. in the asphalt paving, it also may subject the new and recycled asphalt to excess heating.
One known asphaltic plant places a smaller inner drum around the burner at the end of the drum. Aggregate is placed in the smaller central drum and recycled material is placed between the inner and outer drum where it is preheated. This type of asphalt plant cannot process 100% recycled material.
Another known plant utilizes pipes which penetrate the drum heater along its axis and through which the heating gases are made to flow. Aggregate and asphalt are introduced to the drum through one end and the drum is rotated to mix and heat the asphaltic composition. The finished asphalt composition is withdrawn from the opposite drum end. The apparatus includes a heating chamber at one end of the drum, which communicates with the pipes penetrating the interior of the drum. This type of plant may include means for at least a portion of the unburned hydrocarbon volatiles released from the heated composition to be directed into the heating chamber for further combustion. Drums of this type suffer from differential thermal contraction because the drum penetrating pipes are much hotter than the drum. The penetrating pipes which carry gases directly from the combustion chamber can lead to charring of the asphalt. The high temperature of the pipes and the general arrangement of the pipes leads to excessive build-up of asphaltic material which adheres to the pipe surfaces. Drums of this type are difficult to clean because of the limited access to the drum interior.
To rapidly heat large amounts of asphalt paving, current asphaltic plants employ high temperatures to effect rapid heating of the asphalt paving mixture. The use of high temperature leads to the undesirable results of asphalt charring or smoking. Existing drum heaters cannot supply the large quantity of asphalt paving without the use of high temperature and the resulting problems.
A further problem with prior art drum mixers is that they are of limited thermal efficiency. In some asphaltic drum heaters, the design causes the air leaving the plant to be hotter than the finished asphalt paving material wherein considerable heat escapes with the exhaust gases.
Known drum mixer are not designed to be easily insulated to prevent the loss of heat from the asphaltic plant.
Further, virgin aggregate and recycled blacktop which are processed by drum mixers contain water which must be evaporated. The evaporation of water in the feed material for asphaltic plants may absorb as much as half the heat used in preparing the asphalt pavement. Prior art drum mixers provide no means for recovering the latent heat required to evaporate this water.
A further problem with prior art drum mixers is that they require a dust collection system such as a bag house, a wet washer or cyclone separator. Further, prior art does not show how to integrate the dust collection system with the asphaltic plant.
The requirement for a separate dust collecting system is particularly burdensome where the asphaltic drum mixer is used as part of a paving train. A paving train is a number of vehicles traveling in train fashion, one after the other, which perform all the necessary steps required for resurfacing a road. The requirement for an additional pollution-control, dust-collecting vehicle in the paving train adds considerable expense, but this problem has not been addressed by prior art drum mixers.
Yet another difficulty with prior art asphalt plants employing drum heaters is that they are not sufficiently adjustable and do not include sufficient sensors to allow the manufacture of asphalt paving to be controlled in an optimal manner through the use of a programmable controller or computer.
A still further problem with prior art drum mixers is that virgin aggregate causes excessive wear of the drum liner before it reaches the section of the drum where asphalt is added which supplies a lubricating effect.
Finally, prior art drum mixers are not adaptable to the use of microwave heating or other forms of radiant energy because they are not enclosed and separated from the surrounding environment.
What is needed is an asphalt plant having a rotating drum which can efficiently and rapidly heat 100 to 600 tons per hour of asphaltic aggregate without subjecting the asphaltic aggregate to excessively high temperatures which result in charring or smoke. Further, an asphalt plant is needed capable of processing 100% recycled blacktop and employing an apparatus for recovering latent heat from the water evaporated from the aggregate. An asphaltic plant is needed which is contained within a chamber and can be adapted for optimal computer control.